Wireless devices such as terminals are also known as e.g. User Equipments (UEs), mobile terminals, stations (STAs), wireless terminals, communication devices and/or mobile stations. Terminals are enabled to communicate wirelessly in a cellular communications network or wireless communication system, sometimes also referred to as a cellular radio system or cellular networks. The communication may be performed e.g. between two terminals, between a terminal and a regular telephone and/or between a terminal and a server via a Radio Access Network (RAN) and possibly one or more core networks, comprised within the cellular communications network.
Terminals may further be referred to as mobile telephones, cellular telephones, laptops, or tablets with wireless capability, just to mention some further examples. The terminals in the present context may be, for example, portable, pocket-storable, hand-held, computer-comprised, or vehicle-mounted mobile devices, enabled to communicate voice and/or data, via the RAN, with another entity, such as another terminal or a server.
The cellular communications network covers a geographical area which is divided into cell areas, wherein each cell area being served by an access node such as a base station, e.g. a Radio Base Station (RBS), which sometimes may be referred to as e.g. “eNB”, “eNodeB”, “NodeB”, “B node”, or Base Transceiver Station (BTS), depending on the technology and terminology used. The base stations may be of different classes such as e.g. macro eNodeB, home eNodeB, micro eNodeB or pico base station, based on transmission power, functional capabilities and thereby also cell size. A cell is the geographical area where radio coverage is provided by the base station at a base station site. One base station, situated on the base station site, may serve one or several cells. Further, each base station may support one or several communication technologies. The base stations communicate over the air interface operating on radio frequencies with the terminals within range of the base stations. In the context of this disclosure, the expression Downlink (DL) is used for the transmission path from the base station to the mobile station. The expression Uplink (UL) is used for the transmission path in the opposite direction i.e. from the mobile station to the base station.
In 3rd Generation Partnership Project (3GPP) Long Term Evolution (LTE), base stations, which may be referred to as eNodeBs or even eNBs, may be directly connected to one or more core networks.
3GPP LTE radio access standard has been written in order to support high bitrates and low latency both for uplink and downlink traffic. All data transmission is in LTE controlled by the radio base station.
A Universal Mobile Telecommunications System (UMTS) is a third generation (3G) telecommunication network, which evolved from the second generation (2G) Global System for Mobile Communications (GSM). The UMTS terrestrial radio access network (UTRAN) is essentially a RAN using wideband code division multiple access (WCDMA) and/or High Speed Packet Access (HSPA) for user equipment. In a forum known as the Third Generation Partnership Project (3GPP), telecommunications suppliers propose and agree upon standards for third generation networks, and investigate enhanced data rate and radio capacity. In some RANs, e.g. as in UMTS, several radio network nodes may be connected, e.g., by landlines or microwave, to a controller node, such as a radio network controller (RNC) or a base station controller (BSC), which supervises and coordinates various activities of the plural radio network nodes connected thereto. This type of connection is sometimes referred to as a backhaul connection. The RNCs and BSCs are typically connected to one or more core networks.
Specifications for the Evolved Packet System (EPS), also called a Fourth Generation (4G) network, have been completed within the 3rd Generation Partnership Project (3GPP) and this work continues in the coming 3GPP releases, for example to specify a Fifth Generation (5G) network. The EPS comprises the Evolved Universal Terrestrial Radio Access Network (E-UTRAN), also known as the Long Term Evolution (LTE) radio access network, and the Evolved Packet Core (EPC), also known as System Architecture Evolution (SAE) core network. E-UTRAN/LTE is a variant of a 3GPP radio access network wherein the radio network nodes are directly connected to the EPC core network rather than to RNCs. In general, in E-UTRAN/LTE the functions of an RNC are distributed between the radio network nodes, e.g. eNodeBs in LTE, and the core network. As such, the RAN of an EPS has an essentially “flat” architecture comprising radio network nodes connected directly to one or more core networks, i.e. they are not connected to RNCs. To compensate for that, the E-UTRAN specification defines a direct interface between the radio network nodes, this interface being denoted the X2 interface.
Multi-antenna techniques can significantly increase the data rates and reliability of a wireless communication system. The performance is in particular improved if both the transmitter and the receiver are equipped with multiple antennas, which results in a Multiple-Input Multiple-Output (MIMO) communication channel. Such systems and/or related techniques are commonly referred to as MIMO systems.
A Wireless Local Area Network (WLAN) is a wireless communications network that links two or more communications devices using a wireless distribution method, such as spread-spectrum or OFDM radio, within a limited area such as a home, school, computer laboratory, or office building. This gives users the ability to move around within a local coverage area and yet still be connected to the communications network. A WLAN can also provide a connection to the wider Internet.
Most modern WLANs are based on the Institute of Electrical and Electronics Engineers (IEEE) 802.11 standards and are marketed under the Wi-Fi brand name.
The IEEE 802.11 is a set of Media Access Control (MAC) and Physical layer (PHY) specifications for implementing WLAN communication in the 900 MHz and 2.4, 3.6, 5, and 60 GHz frequency bands. They are created and maintained by the IEEE LAN/MAN Standards Committee (IEEE 802).
Ultra-Lean System Design of NeXt Generation (NX) Communications Systems
One design principle currently under consideration for the NX Generation communications systems, also known as Next Radio (NR) in 3GPP context, is to base it on an ultra-lean design. For example, this implies to avoid as much as possible transmissions of so called “always on signals” from the communications network. Expected benefits from such design principle as compared to communications network comprising transmissions of always on signals are expected to be significantly lower energy consumption in the communications network, better scalability, e.g. a larger number of users may be supported within a given area, higher degree of forward compatibility during the RAT evolution phase, lower interference from system overhead signals and consequently higher throughput in low load scenario, and improved support for user centric beam-forming.
Heavy Use of Beam-Forming, e.g. Massive Beam-Forming
Advanced Antenna Systems (AAS) is an area wherein technology has advanced significantly in recent years and wherein a rapid technology development in the years to come is foreseen. Hence it is natural to assume that advanced antenna systems in general and massive MIMO transmission and reception in particular will be a cornerstone in a future NX communications network.
Mobility Reference Signals (MRSs)
In deployments with large antenna arrays and many possible candidate beam configurations, all beams cannot transmit reference and measurement signals in an always-on, static manner for the sake of mobility measurements. Instead, the connected Access Nodes (ANs) select a relevant set of mobility beams to transmit when required. Each mobility beam carries a unique Mobility Reference signal (MRS) in the downlink. A wireless device operating in the communications network is then instructed to measure on an MRS, e.g. a configured MRS, and report the result to the communications network, e.g. to a serving AN. Based on some criteria, for example a difference between an MRS strength between two mobility beams, a handover may be triggered. For mobility to work efficiently, the involved ANs, e.g. the serving AN and one or more target ANs, need to maintain beam neighbor lists, exchange beam information, and coordinate MRS usage.
In the case of uplink measurement based mobility procedure, the wireless device is configured with UL sync (synchronization) signal resources, such as Uplink Synchronization Signals (USSs) that are transmitted by the wireless device in the UL and is listened to by one or several ANs. Based on the received USS's signal strength and some further calibration based on the node capabilities in terms of beamforming, a handover decision and/or a beam-switch decision will be taken.
Both the MRS, e.g. the mobility related reference signal in the DL, and the USS, e.g. the mobility related reference signal in the UL, are expected to be transmitted on-demand and not all the wireless devices are configured to measure on it all the time. It is also to be noted that the amount of unique sequences that are available for MRS and USS are limited to the order of approximately 170 and approximately 70, respectively. Considering that one will have many beams, e.g. hundreds of beams, being transmitted from the same AN, the allocation of MRS may be problematic and also, the allocation of unique USS resources for each wireless device in a densely packed cell having frequent mobility event may be problematic.
As mentioned above, there exist approximately 70 unique USS sequences that may be detected orthogonally by the ANs. Adding time-frequency separation will further increase the orthogonal space for allocating the USS resources but it will also mean that many ANs need to ‘reserve’ these resources for their respective usage. Therefore, it is to be noted that though the USS resources may also be spread in a time-frequency grid, in a dense AN serving many wireless devices, such an allocation may ‘waste’ many UL resources and thus potentially causing a drop in performance.
In an Uplink-based HandOver (UHO) procedure, each wireless device that is under consideration for HO, is requested to perform uplink sounding that is followed with a synchronization with a target AN's Mobility Reference Signal (MRS). This procedure is illustrated on a high level in FIG. 1. As schematically illustrated, a source AN, e.g. a serving AN, transmits a respective measurement control configuration denoted Meas. Control 1 and Meas. Control 2, to a first wireless device denoted UE1 and a second wireless device denoted UE2 operating in a communications network and being served by the source AN. The respective measurement control configuration configures the first wireless device UE1 to transmit a first uplink sounding reference signal denoted Uplink sounding 1 in FIG. 1, and the second wireless device UE2 to transmit a second uplink sounding reference signal denoted Uplink sounding 2 in FIG. 1. The first and second uplink sounding reference signals are received by the source AN and by a target AN. Based on the received first and second uplink sounding reference signals the source AN makes a handover decision. If the source AN decides that the first and second wireless devices are to be handed over to the target AN, the source AN instructs the target AN to perform the handover whereupon the target AN transmits a respective MRS synchronization signal, denoted MRS synch 1 and MRS synch 2, respectively, to the first and second wireless devices.
Thus in an Uplink-based HandOver (UHO) procedure the wireless devices that are in need of HO need to transmit uplink sounding signals, e.g. to transmit the uplink Sounding Reference Signals (SRSs) to one or more ANs operating in the communications network. The Uplink SRS is used by the respective AN to evaluate the channel quality of an uplink path between the wireless device and the respective AN, and to evaluate the uplink timing transmission. As previously mentioned, the transmission of uplink sounding signals is limited by the total number of orthogonal uplink sounding sequences. If there are many wireless devices in need for HO, the procedure will be limited. Also, since there are several other procedures that depend on uplink sounding, the UHO will be limited even for a few wireless devices if other wireless devices exist that have allocated uplink sounding sequences for other purposes. This is a very limiting scaling problem in the number of wireless devices in a cell served by a node that an UHO procedure has.